1. Field of the Invention
The present invention relates to a method of producing a highly pure glass tube. More particularly, it relates to a method of producing a highly pure glass tube having a smooth inner surface.
2. Description of the Prior Art
In the production of glass preforms for optical fibers, several methods have been proposed which comprise depositing fine particles of glass on a surface of a seed member which is in the form of a rod or a tube, drawing the seed member out of the deposited glass soot particles to form a hollow soot preform, and then heating and vitrifying the hollow soot preform to produce a solid glass preform. According to these conventional methods, because the inner surface of the soot preform should be smooth after drawing out the seed member, some measure must be taken on the seed member in order to prevent damage or peeling of the inner surface of the hollow glass preform. Alternatively, the bulk density of the soot preform near its inner surface must be adjusted so that the seed member is easily removed.
One conventional method, referred to here as the "first conventional method," utilizes a seed member made of a metal (e.g., aluminum) having a relatively large coefficient of thermal expansion. Because the metal seed member shrinks to a larger extent than does the hollow soot preform when it is cooled after deposition of the glass soot particles (a period of over several hours from a high deposition temperature to an ambient temperature), the seed member is easily removed from the deposited glass soot particles.
Another conventional method, referred to here as the "second conventional method," utilizes a seed member made of a ceramic material (e.g., Al.sub.2 O.sub.3 and ZrO.sub.9) highly resistant to corrosion and having a much smaller coefficient of thermal expansion than metal.
According to the first conventional method, during the deposition of the glass soot particles on the seed member, a corrosive gas, such as hydrogen chloride, is generated when the glass-forming raw material comprises SiCl. Such a gas corrodes the seed member.
Although the second conventional method can prevent the corrosion of the seed member, it is difficult to draw the seed member out of the deposited glass soot particles when the difference between the deposition temperature and the ambient temperature is not sufficiently large. In addition, ceramic materials are weak against thermal impulses. Accordingly, the seed member must be preheated before the deposition of the glass soot particles, which results in an increase in production costs. Furthermore, after several times of use, the ceramic seed member gradually cracks due to the heat load.
According to any of the conventional methods, the innermost layer of the hollow soot preform should have a comparatively high bulk density in order to prevent peeling of its inner surface during the drawing of the seed member. To achieve the necessary high bulk density in the innermost layer of the soot preform, the atmosphere near the seed member should be heated at a comparatively high temperature. However, at high temperatures, the deposited glass soot particles adhere strongly to the surface of the seed member so that a seed member made of, for example, quartz glass cannot be drawn without causing cracking or peeling of the glass soot preform.